Spinal fusion cage subsidence and expulsion are frequently issues of concern in spinal fusion surgeries. Restenosis due to subsidence has been often documented. See, for example, Marchi et al., J Neurosurg Spine 19, 110-118, 2013. Four subsidence grades have been developed and shown to correlate with the likelihood of restenosis and associated revision rates. The significant potential of endplate-conforming implants in reducing the likelihood of subsidence has also been considered. See, for example, de Beer et. al., Spine Journal 12, 1060-1066, 2012.
Insufficient contact area and load transfer between the vertebral body and the cage can produce excessive load transfer in specific locations that can lead the cage to settle or subside into the vertebral body. Insufficient contact area or pressure differentials between the cage and the vertebral bodies can also produce micro-motions and/or macro-motions that can increase subsidence and result in cage expulsion from the disc space. It is believed that this insufficient contact area is in part due to the anatomical variability in the curvature of the endplates from level to level and patient to patient. Additionally, low bone mineral density index or overaggressive decortications of the endplate can reduce the strength of the endplate and the ability to transfer load from vertebral body to vertebral body.
To minimize these risks, surgeons carefully prepare the opposing vertebral endplates and typically insert the cage having as large a footprint as possible in order to maximize the contact area. When appropriate, the surgeon also places the cage on the apophyseal rings to provide as much support and load transfer as possible for spinal distraction while ensuring the cage is securely nested within the disc space.
These concerns have also been addressed by modifying the shape of the intervertebral cage. Although some cages have been domed to increase contact area, these are often unable to fit and conform to each disc space due to inherent human anatomical variability.
Other procedures concern the use of preoperative CT- or MRI-derived data to facilitate the manufacturing of patient-specific spinal devices. A significant limitation with these devices and patents is that they assume the correct disc space geometry can be clearly identified prior to surgical intervention for disc space released, FSU decompression and spinal alignment corrections. Additionally, the state of the art does not include patient-specific intra-operatively fabricated cages nor patient-specific intra-operatively assembled cages.
U.S. Pat. No. 5,514,180 (Heggeness) discloses intervertebral devices having fixed shapes for accommodating the defined surface contours of vertebral endplates. A method for quantitatively determining the three-dimensional morphology of vertebral surfaces, particularly vertebral endplates, is also disclosed.